Abstract:

Through the practical application of 2500 kW Harsvert-A06/300 medium voltage variable frequency drive made by Beijing Leader & Harvest Electric Technologies Co., Ltd. in the project of water flooding pumps of oilfield, the paper illustrate the main issues solved in development and application of high-power medium voltage variable frequency drives.

I. Preface

The successful application of 2500 kW Harsvert-A06/300 medium voltage variable frequency drive made by Beijing Leader & Harvest Electric Technologies Co., Ltd. in the reform of water flooding pumps of oilfield with variable frequency drives shows that the related technical problems of parallel connection of power components, heat radiation of equipment, environment cooling and site installation etc. have been solved.

II. Large current output

The rated output current of 2500 kW medium voltage variable frequency drive is 300 A. During the system design, apart from satisfying the requirements of rated output current, it is also required to provide the capability of overload of 120% for 1 minute and bearing the shock of 1.5 rated current without damage of components. Therefore, the selection of power components must be based on the designed peak current, i.e., the required passing current of the power component is 620A. Since IGBT enjoys certain instant over current capability, two parallel connected IGBT of rated current of 300A are used to satisfy the design requirement based on consideration for heat radiation, to achieve uniform current output of IGBT under parallel connection and ensure the reliable and safe operation for long time. The main issues in the design of power units are as follows:
1. The IGBT of same model and same batch are used. Components with completely identical performance parameters, in which the current curve, switching losses etc. are screened, are used in production of power component.
2. The circuit parameters are strictly kept physically identical to eliminate the adverse influence of distributed parameters such as structure installation, heat radiation condition and circuit configuration etc. to practical operation.
3. Low inductance loop design is used to avoid affect of the parasitic inductance of power bus to component safety.
4. The unit control panel generates triggering impulse signals of identical steps to ensure the switching control error of the parallel components <1 ms.
5. More strict anti-interference design is used for driving circuit and electromagnetic radiation tests of main electric loop are performed to ensure no distortion of driving waveform to implement same electrical control parameters.
6. Balancing technique of dynamic current is used to ensure identical operating conditions of IGBT to avoid component characteristic parameter difference from temperature variance leading to component damage and system breakdown from non-uniform current.

III. Heat radiation of equipment

Self-cooling and environment heat radiation of medium voltage variable frequency drives are important for the application of medium voltage variable frequency systems. There is a loss of 2~4%, about power of 100kW, for the equipment of 2500kW operating under full load, in which most loss dissipates in space in the form of heat. Effectively transmit its heat to outside in time and keep the system operating temperature in a allowed range are the key factors for ensuring safe operation of the system.
The heat loss of medium voltage variable frequency drives mainly includes two parts: transformer cabinet and power cabinet. Standard design of dry transformers of H level is used for transformers. The insulator can bear high operating temperature of 180،مC and the temperature increase of transformers may reach to 125K: the rated capacity is the effective capacity for natural cooling condition of 145،مC. The demand of the transformer for operating temperature is not sensitive: as long as the operating temperature is lower than the theoretical value of 145،مC, the transformer will satisfy operating requirements. However, in order to ensure the overload capability of the transformer achieves the designed electrical characteristic, designed operating temperature of 130،مC of B level insulators is used for the cooling system. In other words, the cooling system of the transformer cabinet can satisfy the operating demands of dry transformer of B level. Without doubt, the operating conditions of dry transformer of H level are improved, the operating effectiveness is increased and the heat loss is thus decreased. Since the operating temperature of transformers is higher, its loss will be higher and the effectiveness will be lower.
Based on the practical condition, strict air passage structure is not used in the design of the cooling system of transformers and fans are added at the cabinet top to increase the cooling airflow of the transformer to decrease the temperature. The cooling system ensures the heat balance is reached at 110،مC for the transformer under the environmental temperature of 45،مC. The real operation shows that the maximum temperature increase is 65K under site environment temperature of 11،مC and operating with full load. It is much lower than the design standard and requirements can be satisfied with routine air-cooling design.
The power cabinet is another main heating body for the operation of variable frequency drives. The highest allowed operating temperature (case temperature) of the power component IGBT won،¯t be higher than 85،مC. The excessive high temperature increase may lead to the voltage decrease and heat loss of the component increase and vicious cycle forming may cause heat accumulation in the component, leading to the component burnt for too high temperature. Therefore, heat radiation for power components must be handled properly to ensure the safety and reliability of the system. Previously the unit heat for low power products is rather low, the cooling system may have rather high margin and constraint factors are less. For products of 2500kW, the unit heat of power components increases and the air-cooling power must be considered. Therefore, it is particularly important to bring out the heat in the components effectively and transmit the heat to outside quickly.
First, new power components with less power loss are adopted. Parallel connection of components is used to disperse the heat and increase effective heat-radiating area to bring out the internal heat, ensuring the-air cooling can satisfy the requirement for temperature increase. Second, new soldered radiators of cooling plate with high heat conductivity are used to improve heat-radiating effect. Third, the cooling system is designed reasonably based on the heat-radiating power for unit area to ensure the temperature of the component itself lower than 85،مC under environment temperature of 45،مC. Fourth, grid filtering screen cases are used for the cabinet door to decrease the intake air resistance. New synthetic silk with high toughness and permeability is used for the filtering screen to increase the operating efficiency of the cooling system. Fifth, redundant design is used for the cooling fans to ensure the system may operate without reducing rated power when a fan fails and the system will operate with reduced power but not stop when two fans all fail. Thus, the cooling -system of the power cabinet provides safety and reliability for the equipment at the most extent. Practical site operating tests shows that the highest temperature increase of the power components is less than 23K under 100% load and good operating effect is obtained.During system design, apart from solving the cooling of the equipment itself, it is necessary to consider the site environment heat radiation for the application of 2500kW medium voltage variable frequency drive. Based on the site condition of the project of oil field flooding pumps, the environment cooling system of the variable frequency drive is to be included in the present cooling water system of medium-voltage motors. Calculation shows that the design margin of the present cooling water system can satisfy the cooling requirement of all equipment of the united flooding station with the increasing radiated heat of 2500kW variable frequency drive.
Since the operating temperature of the transformer cabinet and the power cabinet in practical operation is different, while the environment temperature is same, and the power cabinet is rather sensitive to the environment temperature, therefore, the following several points are considered for the design of environment heat exchanging system: 1. The environment heat exchanging is implemented with the cooling system of the equipment itself to simplify the system and reduce the harm of auxiliary equipment failure to the system safety without adding interdependent cooling fans andetc. 2. The heat brought out from the equipment by the cooling air is transmitted to outdoor by the cooling water directly through the air-water heat exchanger. Heating to the environment from the main heat sources is thus reduced. 3. Since the normal operating temperature of the transformer cabinet is distinctly higher than that of the power cabinet, heat exchangers are provided for transformer cabinet and power cabinet respectively to increase the utilization of the equipment. 4. Closed environment design is used for the variable frequency drive room and asbestos insulators with good heat insulation are used for the structure to avoid the heating affect of outdoor high temperature during summer. Heat radiation depends the air-water heat exchanger completely.
The power of the exchanger of the transformer cabinet is 55kW and the power of the exchanger of the power cabinet is 45kW. The temperature of the cooling water is <30،مC and the temperature at the air outlet is less than 38،مC. The operating pressure of the cooling water of the heat exchanger is 0.4Mppa. The designed flow is 30m3/h. The designed rated pressure of the tube lines is 1.6Mpa. The tube diameter is DN80, having high design margin. The safety is better than that of the cooling water system for the high-pressure motors, i.e., the cooling water system of the variable frequency drive room does not lower the operating safety of the whole flooding pump system. The site monitoring data shows that under operating condition of the variable frequency drive with full load, the temperature of the cooling water is 2،مC, the temperature of the return water is 9،مC, the temperature of the power cabinet is 11،مC, the maximum temperature of the transformer cabinet 76،مC, the indoor temperature is 10،مC and the outdoor temperature is -17،مC. The operating temperature of the equipment is much lower than the designed value and both of the system cooling and the environment heat radiation can satisfy the design requirement. The air ¨Cwater heat exchanger has good application effects.

IV. Manufacture technology and equipment volume

Production of 2500 kW medium voltage variable frequency drive has higher demands for the manufacture and inspection technology. First, to ensure high system safety and reliability, technically matured components with stable performance are used. Second, the incoming inspection is strictly performed to ensure the components of same model, same batch and matched performance characteristics are used in production. Third, board components are aged and procedure inspecting for producing and assembling are performed strictly. Details, such as component soldering and screw fixing, are processed carefully. Forth, new material and new technology are applied to decrease the product weight and equipment volume.
Reducing volume and increasing unit power are the direction for the development of the medium voltage variable frequency drive technology. There are more obvious requirements for the volume in technical reform projects and popularization, especially for high-power and super high-power medium voltage variable frequency drives. Problems for transportation, installation and construction related with excessive volume often become the obstacles for technique application. Now, along with improvement of manufacturing technology of dry phase-shift insulated transformers and insulation grade, integrated structure can be used for the dry phase-shift insulated transformers. Volume problem for manufacturing two transformers for technical factors, such as manufacturing technology and heat radiation etc., needs not to be considered and the structure design, heat radiation for power units have been solved, progress has been made in aspects of structure and weight bearing of power units and the power density of equipment has been improved effectively. The dimension of 2500 kW Harsvert-A06/300 medium voltage variable frequency drive made by Beijing Leader &Harvest Electric Technologies Co., Ltd. is only 6300،ن1300،ن2674, and the total weight is about 7400kg. It is half smaller than that of similar products. The structure of the whole machine and the site distribution are also simplified greatly, avoiding the construction problems related with the increased power level.

V. Operation effect

Since the equipment put into operation in December 2004, it has been in good operating condition and received obvious investment profit. The technical level of 2500 kW medium voltage variable frequency drive made by Beijing Leader & Harvest Electric Technologies Co., Ltd. and its capability for design and manufacturing can completely satisfy the practical site application requirements. Tests for the output current waveforms of the variable frequency drive and the harmonic analysis show that application of the equipment had completely reached the predicated design purposes. The current equalization in parallel components in the design of high-power medium voltage variable frequency drives has been solved thoroughly. After the self-cooling system of the variable frequency drive equipped with the air-water heat exchanger, excellent control effects of environment temperature have been obtained.

Figure 1. Waveform of 50Hz output current

Figure 2. Analysis for harmonic contents of output current

VI. Conclusion